This invention relates to a process for enhancing the kinetics of hydrogenation/dehydrogenation of MAlH4 and MBH4 metal hydrides, and more particularly to such a process using mechanomixing for low temperature, low-pressure reversible hydrogen storage.
Application of hydrogen as a fuel is attractive because it generates no polluting emissions. However, this attractive application has been hindered due to volumetric problems of storing hydrogen in gaseous or even liquid forms. Hydrogen storage alloys have been proposed and developed to the extent of commercial use in metal hydride batteries. However, the gravimetric hydrogen storage in alloys is still low and requires high pressure and temperature.
Complex chemical hydrides with hydrogen storage capacity have been proposed, and hydrogen generation from this class of compounds has been demonstrated. Unfortunately, hydrogenation of the decomposed complex chemical hydrides is not straightforward, and remains a scientific challenge.
It has been demonstrated that the decomposition of NaAlH4 occurs with at least distinct thermal signatures. The decomposition byproducts have been proposed and identified in the past. The decomposition steps of NaAlH4 can be summarized as follows:
3NaAlH4(solid)xe2x86x92Na3AlH6(solid)+Al+3H2xe2x80x83xe2x80x83(Eq. 1) 
Na3AlH6(solid)xe2x86x923NaH(solid)+Al=3/2H2xe2x80x83xe2x80x83(Eq. 2) 
xe2x80x83NaH(solid)xe2x86x92Na+1/2H2xe2x80x83xe2x80x83(Eq. 3)
Several research groups have investigated the use of catalysts to enhance hydrogenation of complex chemical hydrides. Although some successes have been achieved, the kinetics and reversible hydrogen capacity of these materials remains very low. In particular, the hydrogen capacity of the materials decays very fast during hydrogenation/dehydrogenation cycles.
Bagdanovic (German patent No. 195-26-434, 1995) describes the catalytic effects of transition metal doping of kinetics and reversibility age of a series of alanates. This reference suggests that NaAlH4 doped with titanium decomposes at a reduced temperature and pressure. Further, it is suggested that the reversibility of the compound is also improved by the titanium doping. Others have also observed the catalytic effects of titanium and zirconium doping.
Zaluski et al, J Alloys Compd. 285, 125 (2000), have reported the kinetics enhancement of NaAlH4 by energetic milling of alanate with carbon. Carbon was mixed with the NaAlH4 and milled prior to any hydrogenation/dehydrogenation. No mixing or milling was conducted during the hydrogenation or dehydrogenation. However, it is also reported that the large amount of carbon may have a negative effect as it reduces the gravimetric percentage of active alanates in the composite. It is believed that the carbon actually poisons the process.
Thus, it would be desirable to provide a process for the reversible hydrogenation/dehydrogenation of metal hydrides that overcomes the problems associated with prior art methods.
The invention includes a process for enhancing the kinetics of hydrogenation/dehydrogenation of complex chemical hydrides using mechanomixing and/or mechanomilling. The mechanomixing makes hydrogenation/dehydrogenation of complex chemical hydrides reversible at much reduced temperatures and pressures. The mechanomilling reduces particle size or grain size of the decomposition byproducts, further increasing surface area and intimate contact of the byproducts. In the process of the present invention, complex chemical hydrides can be utilized as a reversible hydrogen storage media for various applications such as transportation, including fuel cells. The process is simple and inexpensive.
The process according to the present invention utilizes complex chemical hydrides of a variety of different formulas, and most preferably complex chemical hydrides generally having the formula MBH4 where M is at least one selected from the group consisting of Na, Li and K, and where B is at least one selected from the group consisting of the elements in the third column of the periodic table. The invention can be practiced using various mixing and/or milling techniques known to those skilled in the art. The invention can be practiced with real-time mixing during decomposition. Wet milling of the decomposition products is also contemplated as producing similar results. Mixing and/or milling methods other than mechanical are also contemplated useful in the present invention.
The invention includes a process for the dehydrogenation/hydrogenation of a complex chemical hydride comprising: decomposing a complex chemical hydride to produce hydrogen and a plurality of byproducts, and whereby the decomposing of the complex chemical hydride produces a foamy mass; mixing the foamy mass to bring the byproducts in more intimate contact with each other and to produce a mixed byproduct mass of reduced volume; and exposing the mixed byproduct mass of reduced volume to hydrogen so that the hydrogen reacts with the byproducts to produce a complex chemical hydride having greater hydrogen content than the byproducts.
In another embodiment of the invention, the complex chemical hydride includes material having the formula MBH4 where M is at least one selected from the group consisting of Na, Li and K, and where B is at least one selected from the group consisting of the elements in the third column of periodic table.
In another embodiment of the invention, the complex chemical hydride includes material having the formula MBH4 where M includes Na and B includes Al.
In another embodiment of the invention, the mixing of the foamy mass includes moving a metal ball through the foamy mass.
In another embodiment of the invention, the mixing of the foamy mass includes stirring the foamy mass with a stirring rod.
In another embodiment of the invention, the decomposing of the complex chemical hydride comprises heating the complex chemical hydride to a temperature ranging from 50xc2x0 C. to 600xc2x0 C.
In another embodiment of the invention, the decomposing of the complex chemical hydride comprises heating the complex chemical hydride to a temperature ranging from about 100xc2x0 C. to 200xc2x0 C. to produce a first set of the byproducts, and thereafter heating the complex chemical hydride to a temperature ranging from greater than 200xc2x0 C. to 300xc2x0 C. to produce a second set of the byproducts.
In another embodiment of the invention, the decomposing of the complex chemical hydride comprises heating the complex chemical hydride to a temperature ranging from 100xc2x0 C. to 300xc2x0 C.
Another embodiment of the invention includes a process for the dehydrogenation/hydrogenation of a complex chemical hydride comprising: decomposing a complex chemical hydride to produce hydrogen and a plurality of byproducts, and whereby the decomposing of the complex chemical hydride produces a foamy mass, and wherein the complex chemical hydride comprises NaAlH4; mixing the foamy mass to bring the byproducts in more intimate contact with each other and to produce a mixed byproduct mass of reduced volume; and exposing the mixed byproduct mass of reduced volume to hydrogen so that the hydrogen reacts with the byproducts to produce a complex chemical hydride having greater hydrogen content than the byproducts.
Another embodiment of the invention includes a process for the dehydrogenation/hydrogenation of a complex chemical hydride comprising: decomposing a complex chemical hydride to produce hydrogen and a plurality of byproducts, and whereby the decomposing of the complex chemical hydride produces a foamy mass, and without mixing the complex chemical hydride during the decomposing of the complex chemical hydride; mixing the foamy mass to bring the byproducts in more intimate contact with each other and to produce a mixed byproduct mass of reduced volume; and exposing the mixed byproduct mass of reduced volume to hydrogen so that the hydrogen reacts with the byproducts to produce a complex chemical hydride having greater hydrogen content than the byproducts.
Another embodiment of the invention includes a process for the dehydrogenation/hydrogenation of a complex chemical hydride comprising: decomposing a complex chemical hydride to produce hydrogen and a plurality of byproducts, and whereby the decomposing of the complex chemical hydride produces a foamy mass, and mixing the complex chemical hydride during the decomposing of the complex chemical hydride; mixing the foamy mass to bring the byproducts in more intimate contact with each other and to produce a mixed byproduct mass of reduced volume; and exposing the mixed byproduct mass of reduced volume to hydrogen so that the hydrogen reacts with the byproducts to produce a complex chemical hydride having greater hydrogen content than the byproducts.
Another embodiment of the invention includes a process for the dehydrogenation/hydrogenation of a complex chemical hydride comprising: decomposing a complex chemical hydride to produce hydrogen and a plurality of byproducts, and whereby the decomposing of the complex chemical hydride produces a foamy mass; mixing the foamy mass to bring the byproducts in more intimate contact with each other and to produce a mixed byproduct mass of reduced volume; and exposing the mixed byproduct mass of reduced volume to hydrogen at a pressure less than 400 pounds per square inch so that the hydrogen reacts with the byproducts to produce a complex chemical hydride having greater hydrogen content than the byproducts.
These and other objects, features and advantages of the present invention will become apparent from the following brief description of the drawings, detailed description of the preferred embodiments, and appended claims and drawings.